1. Field of the Invention
The present invention generally relates to a mass flow controller verifying system, and in particular to a method of verifying a mass flow controller in a gas piping system having a plurality of influent flow gas lines including mass flow controllers arranged in parallel, wherein the influent flow gas lines are joined together to form one post-confluent flow gas line.
2. Description of the Background Art
In order to supply a mixed gas to a process chamber in a process of manufacturing a semiconductor and the like, there is constituted a gas piping system that comprises a plurality of influent flow gas lines arranged in parallel, each having a mass flow controller, the influent flow gas lines being connected to a gas supply source and the influent flow gas lines being joined together to form one post-confluent flow gas line connected to the process chamber.
In a gas piping system as mentioned above, Patent Literature 1 (reference JP2007-525726A) discloses a diagnostic system for verifying whether or not a mass flow controller provided on each of the influent flow gas lines can execute a flow rate control accurately, as defined to be in conformity with a preset flow rate.
In this system, as shown in FIG. 1, a branch flow line BL is formed to be connected in parallel to a post-confluent flow gas line ML, a large-capacity chamber A1 having a prescribed volume is provided on the branch flow line BL, and a pressure sensor A2 is provided on a downstream side thereof. Thus, the verifying of the mass flow controller 1 is performed based on a pressure measured by the pressure sensor A2. Further, in the post-confluent flow gas line ML, there are provided open-close (opening/closing) valves between the connecting points of the branch flow line BL and in the vicinities of the inlet and outlet of the branch flow line BL, respectively.
By this verifying system A100, a mass flow controller verifying method referred to as a rate of rise (ROR) system is used. Referring to a verifying procedure thereof specifically, when a mass flow controller is verified, only an open-close valve on the inlet side of the branch flow line is opened while the other valves are closed, and the influent gas lines SL, except for an influent flow gas line SL having a target mass flow controller 1 to be verified, are also closed. Then, a prescribed set flow rate is set to the target mass flow controller 1 to be verified so as to introduce a gas into the chamber and increase the pressure. Meanwhile, the change in pressure is measured by the pressure sensor, and a verifying volume corresponding to the volume of the chamber A1 is calculated based on the rate of pressure change. Subsequently, the calculated verifying volume is compared to a reference volume that is a sum of the prescribed known chamber volume and an approximate rough volume of a piping portion from the target mass flow controller to the chamber. In this comparison, when the calculated verifying volume is equal to the reference volume, it is judged that the mass flow controller can execute a flow rate control as defined to be in conformity with the set flow rate. When the calculated verifying volume is not equal to the reference volume, it is judged that the mass flow controller cannot execute a flow rate control as defined to be in conformity with the set flow rate due to clogging and so forth.
However, in the mass flow controller verifying system as disclosed in Patent Literature 1, some problems are involved as described below.
In order to introduce this verifying system into an existing gas piping system for use in a semiconductor manufacturing process and so forth, it is necessary to newly constitute piping, such as a branch flow line connected to a post-confluent piping, and to provide a chamber for obtaining a reference volume. In some cases, however, it may be difficult to newly provide an additional piping and chamber due to a layout of a factory and the like, and due to the resulting increased costs for designing the piping for the verifying system to fit to the existing piping system and installing of the new piping and chamber.
In order to verify a mass flow controller accurately according to the methods of an ROR system and the like, it is necessary to confirm the reference volume correctly. In this verifying system, the sum of the piping volume in a region from a target mass flow controller to be verified to the chamber and the known volume of the chamber is used as the reference volume, and the piping volume is usually roughly calculated based on, e.g., a length of the corresponding piping. However, it is difficult to accurately calculate a volume of a curved portion of a piping and a volume inside an open-close valve and so forth, and therefore the calculated value of the reference volume is not very reliable. Moreover, if it is intended to be able to absorb an error related to such a piping volume, the volume of the chamber should be increased to a certain extent.
If the volume of the chamber is increased to be large or the reference volume is large, an amount of a pressure change required for verifying cannot be measured without a necessary long time lapse. Therefore, it takes a long time to verify each of the mass flow controllers.